This invention relates to a control for direct current motors utilizing silicon controlled rectifiers (SCr's) to supply power to the motor and in particular to a circuit for safeguarding against shorting or commutation failure of the main SCR.
It is well known that the direct current supplied to a motor from a constant potential power source such as a battery may be selectively varied by controlling the average power to the motor and that a solid state SCR can be used as a switching device to repeatedly connect and disconnect the battery to and from the motor. The power supplied to the motor is determined by the ratio between the time the SCR is turned on and conducts and the time the SCR is turned off and is non-conducting. The greater this ratio of conduction to non-conduction, the more power there will be to the motor. Typically in vehicles having a direct current propulsion motor and an SCR control therefor, the ratio of conduction to nonconduction is varied by adjusting the frequency at which the main SCR is turned on by means of a pulse generator whose rate is controlled in response to a foot-actuated accelerator pedal.
In operation there are a number of malfunctions which can occur in an SCR control, and this invention is related to two of these malfunctions.
First, the main SCR may short so that it cannot be commutated, or turned off. In such case, the ratio of conduction to non-conduction becomes and stays at a maximum (since there is no non-conduction), full power is delivered to the motor and the operator loses control. It is thus desirable that a shorting of the main SCR be detected and the motor disabled so that further operation of the vehicle is suspended until the defective SCR is replaced.
Secondly, during operation the commutation circuit can occasionally fail to commutate the main SCR and the main SCR stays on. Very often the fault in the commutation circuit will clear itself if the power to the main SCR circuit is turned off and then on again and normal cycling of the main SCR is resumed.
A commutation failure which does not turn the main SCR off has the same immediate result as a shorted SCR, namely, the current to the motor is now continuous.
Thus, a fault detecting system should distinguish between a shorting of the main SCR (wherein the SCR is incapable of being turned off) and a commutation failure (wherein the SCR is capable of being turned off but is not). If the main SCR is shorted, the motor should be disabled. In the latter event, the fault-detecting system should cause commutation of the SCR and then allow operation to continue, without attention by the operator, if the commutation circuit has cleared itself. If the commutation circuit continues to malfunction then the motor should be disabled so that the defect can be repaired.